1. Field of the Invention
The present invention relates to a carrier regeneration circuit, and more specifically, to a carrier regeneration circuit for regenerating the carrier for demodulation in a quadrature partial response modulation system.
2. Description of the Prior Art
In the field of data transmission, partial response systems are well known. Namely, by transmitting the symbols by means of the waveform having a constant intersymbol interference in one bit or more, frequency spectrum of the response waveform can be reduced, thus transmission bandwidth can be saved. Such method is called the duo binary, and its general method is the partial response system. For example, this partial response system is described in the papers: "Generalization of Techniques for Binary Data Communication" by E. R. Kretzner, IEEE Transactions on Communication Technology, April 1966, Volumn COM-14, No. 1, pp 67-68; and in "Data Transmission Using Controlled Intersymbol Interference" by K. H. Schmidt, Electrical Communication Volume 48, No. 1 and 2, 1973.
In class 1 partial response systems, an input pulse is converted into a pulse of a doubled width and accordingly preceding and succeeding pulses are superimposed. Thus, a 2-level digital input signal becomes a 3-level signal.
In such a partial response system, it is necessary to accumulate the preceding element (bit) group in order to demodulate the signal which was subjected to partial response conversion, thereby to return to the initial 2-level signal. Because of this, a problem arises in that erroneous propagation occurs during demodulation. Thus, symbol processing, called precoding, is performed before the partial response conversion. To amplify, for a 2-level digital signal having the levels "0" and "1" and converted in accordance with partial response conversion to the levels of "0", "1", and "2", for example, the code conversion is always performed so that the level "1" of the converted, partial response signal corresponds to the level "1" of the initial 2-level input digital signal.
Quadrature amplitude modulation systems as well are known in the prior art. In such systems, quadrature-related carriers, i.e., carriers of the same frequencies but having a phase difference therebetween of 90.degree., are independently amplitude modulated and then combined into a QAM (Quadrature Amplitude Modulation) signal. An ordinal 4 phase PSK (Phase Shift Keying) modulation system is a kind of QAM system. The configuration of the quadrature amplitude modulation system is shown, for example, in FIG. 8 of the U.S. Pat. No. 3,806,807.
Demodulation of a QAM signal at the receiver, or receiving side of this system, is performed by regenerating the carrier and synchronously detecting the received QAM signal with the regenerated carrier.
In this case, when the phase of the regenerated carrier skips to a different stable phase, there results an error in the demodulated data. The fact that the phase of the regenerated carrier is not determined precisely as one specific, absolute phase, is generally referred to as phase ambiguity of the regenerated carrier.
Methods have been proposed whereby demodulation can be achieved correctly, in spite of phase ambiguity of the regenerated carrier, through use of differential encoding, by modulating two carriers for quadrature amplitude modulation with the signal which has been subjected to the partial response conversion. In this case; it is necessary that the phase ambiguity of the regenerated carrier is a multiple of 90.degree..
Reference is made at this juncture to the concurrently filed application of Tadayoshi Katoh, U.S. application Ser. No. 716,098 entitled PARTIAL RESPONSE, QUADRATURE AMPLITUDE MODULATION SYSTEM, the teachings of which are incorporated herein by reference.